1. Field of the Invention
The present invention relates to a modulation format identification device for identifying a modulation format of a received signal transmitted based on one modulation format among a plurality of different digital modulation formats and a method of the same.
2. Description of the Related Art
Where transmitting symbolized information by an electromagnetic wave (wirelessly), a modulation operation for multiplying a so-called baseband signal with a high frequency sine wave becomes necessary.
The high frequency sine wave to be multiplied with the baseband signal is referred to as a carrier. When precisely expressing this sine wave mathematically, it can be indicated as follows by using the three elements of an amplitude a, an angular frequency xcfx89, and a phase xcex8.
y=axc2x7cos(xcfx89t+xcex8)xe2x80x83xe2x80x83(1)
As the digital modulation format, by changing one of these three elements of the amplitude a, angular frequency xcfx89, and the phase xcex8, three basic types of modulation formats can be taken.
Namely, there are the three modulation formats of amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (ASK).
Among these, ASK suffers from a problem in the threshold voltage. FSK can solve this problem of the threshold voltage, but in contrast suffers from a problem in terms of the effective utilization of frequency resources since the occupied band width becomes larger than with ASK.
Contrary to this, in PSK, the same level of occupied band width as ASK can be maintained, the problem of the threshold voltage can be solved, and an error characteristic better than the ASK can be achieved with the same S/N ratio.
In the above basic modulation formats, one bit of digital information is transmitted by using two types of waveforms, but it is not necessary to limit the waveforms used for transmission to two types. It is also possible to send a plurality bits of information at one time by using a plurality of waveforms.
Therefore, a so-called M-ary modulation format for sending a larger amount of information at one time with the same transmission band width is generally used.
As such M-ary modulation formats, corresponding to the basic modulation formats, there are an M-ary amplitude shift keying (M-ASK), an M-ary frequency shift keying (M-FSK), and an M-ary phase shift keying (M-PSK).
Among them, M-ASK is suitable for a system wherein saving of the frequency resources is more important than a transmission signal power, while M-FSK is suitable for a communication system wherein there is leeway in the frequency resources, but there is a strict restriction in the transmission power or the like.
M-PSK is advantageous in comparison with the other modulation formats in terms of the effective utilization of the power and frequency, so has been widely applied to communication systems etc.
In M-PSK, the phase of the carrier to be transmitted is finely controlled. 0 degree and 180 degrees are used in binary PSK, but the phase value is set for every 360xc2x0/M in M-ary (M=2, 4, 8, 16, 32, . . . ) PSK. The larger the value of M, the narrower the occupied band width.
Note that the quadrary PSK modulation format in M-PSK is referred to as quadrature PSK (QPSK).
In this M-PSK, a large amount of information can be sent without waste up to the binary, quadrary, and hexary formats, but there is considerable waste when viewed from the signal constellations in 16-ary PSK or higher PSK.
Therefore, there is known the quadrature amplitude modulation (QAM) format designed to enable a large amount of information to be sent without waste in a transmission system restricted in transmission band width by special constellations of the signals.
QAM is an indispensable modulation technique in a system where a large amount of information must be sent with a narrow transmission band width.
Accordingly, in place of for example 16-ary PSK and 32-ary PSK, 16-ary QAM and further 32-ary QAM etc. are used.
Summarizing the problem to be solved by the invention, in recent years, a software-defined radio (SDR) device operating over a wide frequency band and a variety of digital modulation formats such as M-PSK or M-QAM have been proposed (refer to for example J. Mitola, xe2x80x9cSoftware Radioxe2x80x9d, IEEE Comm. Magazine, May 1995).
In this case, the receiving side must identify the modulation format of the received signal transmitted based on one modulation format among the plurality of different modulation formats.
As the conventional method, there is the method of determining in advance the modulation format of the signal to be received on the receiving side, but there is a problem from the viewpoint of practical use.
Further, as another method, there is a method of automatically identifying the modulation format by using more complex statistics or transformation in the frequency domain by fast fourier transforms (FFT), but there are the disadvantages that the configuration becomes complex etc.
Further, in recent years, a device for identifying the modulation format of the received signal on the receiving side has been proposed, but it has been proposed as a special circuit which can be applied for only the identification of a specific modulation format. No device for identifying the modulation format of the received signal irrespective of the modulation format has ever been proposed.
An object of the present invention is to provide a modulation format identification device capable of realizing a practical receiver capable of identifying the modulation format of a received signal irrespective of the modulation format by a simple configuration and a method of the same.
According to a first aspect of the present invention, there is provided a modulation format identification device for identifying a modulation format of a received signal transmitted based on one modulation format among a plurality of different modulation formats, comprising a primary decision circuit having a plurality of detectors provided corresponding to the plurality of modulation formats and receiving as input the received signals in parallel thereto, counting a number of symbols in accordance with each modulation format for every detector, making a primary decision that the received signal has been modulated by the related modulation format when a count exceeds a constant threshold value and outputting these decision results from the detectors and a logic circuit for exclusively selecting one modulation format upon receipt of a plurality of output results of the primary decision circuit.
Preferably, in the device of the present invention, each detector comprises a hit counter for counting the number of symbols in each in-lock area designated in advance for exactly a counting period determined in advance and a detection flag generation circuit for comparing the count output from the hit counter and a threshold value set in advance and outputting a detection flag when the number of symbols is larger than the threshold value, and the logic circuit selects one modulation format based on a plurality of detection flags from the detection flag generation circuit.
According to a second aspect of the present invention, there is provided a modulation format identification method for identifying a modulation format of a received signal transmitted based on one modulation format among a plurality of different modulation formats, comprising the steps of counting the number of symbols in accordance with each modulation format and making a primary decision that the received signal has been modulated by the related modulation format when a count exceeds a constant threshold value and exclusively selecting one modulation format upon receipt of a plurality of primary decision results.
Preferably, in the method of the present invention, in the primary decision, counting the number of the symbols in an in-lock area designated in advance for exactly a counting period determined in advance and deciding that the received signal has been modulated by the related modulation format when the count is larger than a threshold value set in advance.
Further, in the device and method of the present invention, preferably the modulation signal is a signal capable of expressing an in-phase component and a quadrature component in an orthogonal coordinate system and having symmetry in the orthogonal coordinate system, and the primary decision is made based on the number of symbols in one quadrant among four quadrants of the quadrature coordinate system.
Further, in the present invention, as the threshold value, preferably a common value is given with respect to the modulation formats.
Further, in the present invention, preferably the threshold value is set in accordance with the channel condition for every modulation format.
According to the present invention, for example a signal modulated by a predetermined modulation format is input to the primary decision circuit.
The primary decision circuit counts the number of symbols of the received signal in accordance with the modulation format for every detector.
Then, this count and a threshold value set in advance are compared. When the result is that the count exceeds a constant threshold value, it is primarily decided that the received signal has been modulated by the related modulation format.
The result of the primary decision of the detectors is input to the logic circuit.
Then, the logic circuit selects one modulation format exclusively from among a plurality of primary decision results.